Valve terminology varies according to the industry (e.g., pipeline or oil field service) in which the valve is used. In some applications, the term “valve” means just the moving element, whereas in other applications, the term “valve” includes the moving element, the valve seat, and the housing that contains the moving element and the valve seat. To clarify the following description of the present invention, a valve suitable for abrasive fluids, such as oil field drilling mud, comprises a valve body (the moving element) and a corresponding valve seat.
The valve body incorporates an elastomeric seal within a peripheral seal retention groove. These valves are usually mounted in the fluid end of a high-speed pump incorporating positive displacement pistons or plungers in multiple cylinders. Such valves are frequently web-seat, stem-guided designs or full open seat designs adapted for high pressures and repetitive impact loading of the valve body and valve seat. Both leakage and premature failure due to metal fatigue must be overcome in designing these valves, and special attention is focused on the moving element (or valve body).
The valve body element typically comprises at least one groove for a peripheral elastomeric seal. If preformed seals are to be used, such a groove requires finish machining to closely match the dimensions of seals like the “snap-on” type or seals secured with a removable seal retention plate. This finish machining may be reduced or eliminated if elastomeric seals are cast and cured in place (herein “cast-in-place”) in their groove. Further, cast-in-place seals may be mechanically locked to a valve element by forming them over interengaging or interlocking (herein “interdigitating”) projecting-receiving formations on the element. Such interdigitating of valve element and seal has become a recommended structural feature of cast-in-place seals that was evidently difficult or impossible to achieve with “snap-on” type seals. See, for example, the entire U.S. Pat. No. 4,860,995 (incorporated herein by reference), particularly col. 7, lines 17-57.
But manufacture of valve elements with interdigitating cast-in-place seals, such as those described in the '995 patent has historically involved added costs. These added costs arose because, as described in the '995 patent, the seals are preferably bonded to a valve element to increase its overall integrity. See the '995 patent, col 7, lines 47-50.
Even though the manufacturing cost of valve bodies for bonded cast-in-place seals is almost identical to the analogous cost of valve bodies for “snap-on” seals, the added cost of preparing the valve body for bonding increases the cost of the valve to the point that these valves have not been competitive on price. The added costs of bonding include cleaning the valve groove of all oil and contaminants, applying a bonding adhesive, and storing the valves in a low-humidity, dust-free environment while the valves await casting, bonding, and curing of the urethane on the valve body.
Despite their added cost and, as noted in the Detailed Description herein, their unexpectedly short service life, bonded cast-in-place seals have achieved limited acceptance on valve bodies having circular “Channel-Beam” sections or other one-piece valve bodies. Channel-Beam valve bodies are characterized by a forged bowl shape as seen, for example, in FIG. 1 of U.S. Pat. No. 5,249,600 (the entire '600 patent is incorporated herein by reference). The acceptance of bonded cast-in-place seals in these applications, notwithstanding their relatively high cost and shortened service life, may be attributed in part to the benefits of the Channel-Beam design itself and in part to the problems of using “snap-on” type seals on this type of valve body.
Forged Channel-Beam valve bodies such as those in the '600 patent have exceptional stiffness, strength, and resistance to fatigue failure. But, in addition to the above problems with bonded cast-in-place seals, they also have several disadvantages when used with “snap-on seals. First, rough valve body forgings in the Channel-Beam shape require substantial material removal for finish machining of the integral seal retention groove if a “snap-on” seal is to fit properly. Second, even if an accurately preformed elastomeric seal in the shape of a toroidal ring is snapped into a carefully machined seal retention groove, it may not fully seat within the groove due to stack-up (i.e., additive effects) of manufacturing tolerances. The resulting out-of-round seal condition can cause early valve failure. Similar disadvantages are also seen in other Channel-Beam designs, such as those described in U.S. Pat. Nos. 3,191,617; 3,202,178; 3,742,976; 4,180,097; 5,345,965; and 5,431,186, all incorporated herein by reference.
Thus, notwithstanding their relatively high cost and/or problems with limited service life, valve bodies having “snap-on” or cast-in-place seals in a seal retention groove analogous to the one-piece Channel-Beam design have gained limited industry acceptance. Although the seals have been problematical, the strength and stiffness of the Channel-Beam shape tend to reduce valve body distortion about one or more radial axes (i.e., axes radiating perpendicularly from the valve body's longitudinal axis of symmetry). This type of distortion is associated with both leaks and fatigue failures, and it is particularly likely to occur on valve bodies mating with web seats.
Metal fatigue is induced when cyclical high pressure is applied to a valve body sealed against, and being distorted by, a web seat. The pressure tends to repeatedly force the disc-shaped area of the valve body (i.e., the flange) into the spaces between the seat webs. This distortion, plus impact loads secondary to valve closure and bending moments caused by pressure on the valve seal, causes non-uniform stresses in the flange, particularly in the two opposing walls of the flange's peripheral seal retention groove.
For example, the groove wall closest to the valve seat typically sustains relatively high impact load stress transmitted through a peripheral metal sealing surface (i.e., a valve body impact area) for contacting the seat when the valve closes. This area of the flange, then, tends to be particularly susceptible to peripheral metal fatigue failures. In contrast, the flange area comprising the opposite groove wall tends to be bent longitudinally through force exerted by the seal within the groove when the seal is displaced as the valve fully closes. This bending stress is minimal peripherally and greatest in that part of the wall nearest the valve body's longitudinal axis of symmetry, that is, the central part of the wall (i.e., that part of the wall nearest the base of the groove).
Peripheral flange stresses (and the associated fatigue failures) may be reduced to a limited extent, and valve sealing improved, by a properly-placed peripheral elastomeric seal which contacts the valve seat on closure. Improper placement of this seal, however, leads to an out-of-round condition that may actually increase leaks and hasten valve failure. Leaks may occur, for example, past an out-of-round “snap-on” seal or around a cast-in-place seal that shifts during use within its seal retention groove.
The potential advantages of an elastomeric seal insert (including extended valve service life and improved valve performance) are commonly reduced or lost entirely if the seal is poorly fitted to the valve body or if the seal shifts in its retention groove during use. Both of these conditions allow leaking high-pressure fluid to jet through one or more leak paths. If the valve remains in service, each jet of high-pressure fluid will literally wash away a portion of the hardened steel of the valve body and/or seat. Multiple and near-simultaneous failures of this kind may give a valve body flange the appearance of a wrinkled cupcake paper.
Further, leaks due to displacement of cast-in-place elastomeric seals often occur secondary to failure of the elastomer adjacent to the special adhesive that bonds the seal to the groove. If a portion of the seal is tightly bonded to the groove wall, background stress within the seal elastomer will increase as the elastomer cures because the seal as a whole tends to shrink away from the walls to which it is bonded. Deleterious effects of this background elastomer stress (i.e., elastomer stress that is present even when the seal is not in contact with a valve seat) may be significantly aggravated when a valve body closes against its seat. The resulting elevated internal elastomer stress tends to reduce the service life of the seal by predisposing it to cracking, tearing and/or extrusion. FIG. 1A schematically illustrates an example of such stress-induced tearing in an elastomeric seal on a valve body shaped similarly to that in FIG. 2 of the '995 patent.
By including a schematic representation of a valve seat, FIG. 1A also shows the proximity of a stress-induced tear with the portion of an elastomeric seal that would be subject to extrusion stress as the valve body mates with the valve seat. The requirement that a valve seal make contact with the valve seat prior to metal-to-metal contact between the valve body and seat means that the seal elastomer is subjected to strong compressive forces as the valve is closing. Since movement of the seal against the valve seat is restricted by friction, by the metal portion of the valve body, and by the valve seat itself, a portion of the seal elastomer tends to be extruded into the (schematically shown) extrusion gap. Thus a portion of the seal tends to be repeatedly deformed by this extrusion process each time the valve closes. Elastomer stress due to this deformation can thereby add to the background elastomer stress noted above to precipitate premature failures like the illustrated tear. On the other hand, reductions in background elastomer stress can reduce overall elastomer stress during valve closing, thereby lengthening the service life of the seal.
To avoid the extra cost of adhesive and the induced background elastomer stress associated with curing of cast-in-place adhesively-bonded seals, a deformable seal may be stabilized in a groove by threaded serrations directly opposing each other on the sides of a seal retention groove. Stabilization of this general type is termed mechanical locking in the '995 patent. Serrations for this purpose are illustrated in U.S. Pat. Re. Nos. 29,299 and 4,676,481 (both patents incorporated herein by reference). While effective for limiting seal movement within the groove, serrations arranged in this manner can themselves raise stress in both the groove walls and the elastomer. And any unnecessary increases in stress will predispose the elastomer seal to premature failure.
Premature stress-induced failures, however, would be relatively rare in the valves illustrated and described in the '299 and '481 patents. These are pipeline valves that are intended for applications with a maximum total valve service life of about 1000 open-close cycles. In contrast, stem-guided valves of web-seat or open seat design that are suitable for drilling mud commonly operate at relatively high pressures and complete two open-close cycles per second. Service life for these valves is measured in millions of cycles (commonly about 1 to 10 million open-close cycles). Incipient valve body fatigue failures that are not manifest after only 1000 open-close cycles become a significant maintenance problem in a valve experiencing millions of cycles during its expected service life.
For the latter valves, a more robust seal assembly is needed, comprising an elastomeric seal that is cast-in-place on a valve body having an integral seal retention groove. Use of a separate adhesive for bonding the seal within the groove, although taught in the '995 patent, should in fact be avoided to reduce background stress in the cured seal elastomer. Further, serrations to retain the seal in the groove should be placed to minimize their effects as stress raisers for the valve body, with consideration being given to the different stress distributions in the opposing groove walls.
Such serration placement would minimize valve body fatigue failures due to impact loads and bending stress. Further, serrations should be designed so the seal elastomer can retain effective contact with the serrations on the groove walls while it is being cured. Such continued effective contact (i.e., interdigitation) would ensure that, notwithstanding shrinkage of the elastomer during curing, the seal could be retained in its groove without suffering displacement that would materially reduce its service life.